In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures on the order of 1400° C. or higher. If exposed to normal atmosphere, it would suffer very rapid scaling due to oxidation at such high temperatures. Therefore, a sealed enclosure is provided beneath the casting rolls to receive the hot strip and through which the strip passes away from the strip caster, the enclosure containing an atmosphere which inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust gases which may be reducing gases. Alternatively, the enclosure may be sealed against ingress of oxygen containing atmosphere during operation of the strip caster. The oxygen content of the atmosphere within the enclosure is then reduced during an initial phase of casting by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
In twin roll casting, eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls, or due to distortion and wear when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls, and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or tertiary fluctuations within the generally sinusoidal patter. In accordance with embodiments of the present invention, these repeated thickness variations can be reduced significantly by individually driving the rotation of the casting rolls and adjusting the angular phase relationship between the rotation of the casting rolls to reduce the effect of the eccentricity in the rolls on the variation in profile of the cast strip. One way of compensating for this problem is described in U.S. Pat. No. 6,604,569, issued Aug. 12, 2003.
Described herein is a method of producing thin cast strip by continuous casting that comprises the steps of:                (a) assembling a twin-roll caster having a pair of casting rolls forming a nip between the casting rolls;        (b) assembling a drive system for the twin-roll caster capable of individually driving the casting rolls and maintaining an alignment angle between the casting rolls;        (c) assembling a metal delivery system capable of forming a casting pool between the casting rolls above the nip and having side dams adjacent an end of the nip to confine the casting pool;        (d) introducing molten metal between the pair of casting rolls to form a casting pool supported on casting surfaces of the casting rolls and confined by the side dams;        (e) counter-rotating the casting rolls to form solidified metal shells on the surfaces of the casting rolls and to cast strip from the solidified shells through the nip between the casting rolls; and        (f) modifying the alignment angle between the rotating casting rolls such that eccentricities between the casting rolls are reduced to form cast strip having a more uniform thickness.        
In addition, sensors may be provided which are capable of sensing eccentricities in casting surfaces of at least one of the casting rolls and generating electrical signals indicating variation in such eccentricities of the casting roll(s). Also, a controller is provided which is capable of varying the alignment angle in rotation to reduce a variation in shape of the strip due to the eccentricities in the casting rolls.
Also described as part of the invention is a twin-roll casting apparatus for producing thin cast strip that comprises:                (a) a pair of casting rolls positioned laterally adjacent each other to form a nip between the casting rolls through which metal strip may be continuously cast;        (b) a drive mechanism for the casting rolls capable of individually driving the rotational speed of the casting rolls in a counter-rotational direction to cause the strip to pass through the nip between the casting rolls; and        (c) a control mechanism capable of varying an alignment angle in rotation between the casting rolls to reduce the effect of eccentricities in the casting rolls on the profile of the strip produced by the casting rolls.        
In addition, the twin-roll casting apparatus comprises sensors capable of sensing eccentricities in the casting surfaces of the casting rolls and generating electrical signals indicating variations in eccentricity in the casting surfaces of at least one, and typically both, of the casting rolls. The control mechanism is capable of varying the alignment angle in rotation between the casting rolls to automatically reduce effects on the profile of the strip from the eccentricities in the casting rolls in response to the electrical signals.
Other details, objects and advantages of the invention will be apparent from the following description of particularly presently contemplated embodiments of the invention proceeds.